1. Field of the Invention
The present invention relates to a memory structure and a method of making the memory structure, and more particularly, to a memory structure having two gates embedded in the substrate and the method of making the same.
2. Description of the Prior Art
It is well known that memory devices have become essential elements of electronic products. For example, a cell phone, a computer system, and a personal digital assistant (PDA) all comprise memory devices to store data or program codes for further data processing. Memory devices can be separated into volatile and non-volatile, where non-volatile memory devices can retain the data even if the power supply is turned off. Unlike non-volatile memory devices, volatile memory devices require power to maintain the stored information.
Flash memory is a non-volatile computer memory that can be electrically erased and reprogrammed. It is a technology that is primarily used in memory cards and USB flash drives, which are used for general storage and transfer of data between computers and other digital products. As electronic devices become smaller, the size of the flash memory cells shrinks as well. Today, in order to make electronic devices of small sizes, increasing the integration of elements is a major issue.
FIG. 1 shows a structure of a two-bit flash memory cell according to prior art. As shown in FIG. 1, the structure of the conventional two-bit flash memory cell comprises: a substrate 10, a gate insulating layer 12 comprising a silicon oxide layer 18, a silicon nitride layer 16, and a silicon oxide layer 14, wherein the silicon nitride layer 16 has two charge storage regions 16a, 16b and a control gate 20 positioned on the gate insulating layer 12.
The two charge storage regions 16a, 16b of the above-mentioned flash memory cell are capable of storing two-bit data. However, as the size of the electronic device shrinks, a short channel effect is formed because the gate channel length becomes shorter. Furthermore, the smaller the size, the more difficult it is to achieve alignment of elements. Therefore, a new structure of the memory cell and a new method of making the memory cell are needed to overcome the short channel effect and solve the problem of element alignment.